This invention relates to an electro-optic rearview mirror system for a vehicle.
Electro-optic rearview mirrors are very well known in the art and include a mirror housing containing a variable reflectance electro-optic mirror unit. While many different constructions of variable reflectance electro-optic mirror units are known, a typical construction uses two substantially parallel glass plates, one of which (herein referred to as the rear plate) is coated on one surface with a reflective layer (the other plate is referred to as the front plate). The front and rear plates are separated by a space which contains an electro-optic medium allowing variation in the amount of light transmitted through the medium by varying the strength of an electric field applied across the medium. For example, in liquid crystal rearview mirrors the space between the front and rear plates is filled with a semi-viscous liquid crystal material. In electrochemichromic or electrochromic mirrors, the space contains a liquid, thickened liquid, gel or semi-solid material.
The mirror unit is mounted in the mirror housing with the front plate facing the viewer, so that light incident externally on the mirror unit returns to the viewer, after reflection at the rear plate, only after a double crossing of the electro-optic medium. Electrode means, for example a respective transparent electrode layer on each plate, permit varying the electrical field across the electro-optic medium, thereby to correspondingly vary the level of reflected light returned to the viewer and accordingly the reflectivity of the mirror unit as a whole.
In the simplest case the reflectivity of the mirror unit is switchable manually between a maximum appropriate for daylight use to a minimum appropriate for nighttime use where it is required to reduce the glare from headlights of following vehicles. However, it is also known to mount one or more light sensors in the mirror housing which sense the light levels forwardly and/or rearwardly of the vehicle and automatically adjust the reflectivity of the mirror unit according to selected criteria. Typical arrangements of the latter kind are described in U.S. Pat. Nos. 4,793,690, 4,799,768, 4,886,960, 5,193,029, 5,140,455 and 5,151,816.
Hitherto, in all cases the control circuitry for the electro-optic mirror unit has been contained, usually on a printed circuit board (PCB), within the mirror housing itself.
However, this has substantial disadvantages. The mirror housing needs to be fairly bulky to accommodate the control circuitry, which is undesirable from an aesthetic point of view, and this is exacerbated by the current trend to located more and more function in the mirror housing such as a compass display, navigational display system and infra-red (IR) sensor for automatic locking system. Also, the control circuitry tends to generate substantial heat, often as high as 2-3 watts, which means the mirror housing has to be designed for adequate ventilation. Further, the mirror is complex to assemble and has a high replacement cost.
It is an object of the present invention to mitigate these disadvantages.
Accordingly, the invention provides an electro-optic rearview mirror system for a vehicle, comprising a mirror housing containing an electro-optic mirror unit, at least one light sensor for sensing the ambient light level outside the vehicle, and control circuitry responsive to signal(s) from the light sensor(s) to establish a drive voltage for the electro-optic mirror unit, wherein the control circuitry is located outside the mirror housing.
Preferably, the mirror housing is mounted interior of the vehicle.
The light sensor(s) may also be located in the mirror housing, but they may alternatively be located elsewhere in the vehicle where an adequate view of the ambient light level exterior of the vehicle can be obtained.
The control circuitry is preferably located in a header, which is a shallow housing mounted on the ceiling of the vehicle, preferably located behind the rearview mirror housing overhead and centrally between the driver and the front passenger seats.
An advantage of the invention is that the mirror housing does not require interior space to be allocated to the control circuitry and therefore the mirror housing can be reduced in size or, if desired, can accommodate extra functions, such as a compass display, navigational display system or infra-red (IR) sensor as referred to above, without unduly increasing the size of the housing.
Further, the heat generated in the mirror housing is reduced, and also the constructional complexity and replacement cost is reduced. Also, the reduction in components in the mirror, and the use of a black seal on the perimeter of the electro-optic element, permits a simpler assembly method to be used.
Preferably the control circuitry shares a processor which is used for other control functions of the vehicle. This has further advantages.
First, it allows additional functions contained in the mirror housing to be connected to a vehicle bus system 44 without the cost and complexity of adding a separate bus connection node in the mirror. Also, switches and indicators presently located in the mirror housing can be grouped in a central location with other controls. Further, there is the ability to have the electro-optic mirror sensitivity adjusted to customer preferences at dealer service intervals. Additionally, sensors and other features located in the mirror housing can be used for multiple functions, for example, the light sensors used for the electro-optic mirror unit can be used for automatic headlight control.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: